Traffic flow is not that different from the flow of anything else. Imagine heat flow along an iron rod or water flowing down a pipe. If you observe the traffic (on a highway) from a far enough distance, the mass of cars would appear analogous to a (slowly) flowing fluid.
Let be the density (substance per unit length) and let be its rate of flow past x at time t.
The mass present in a segment of length is:
The rate of flow of this mass